The cheapest option for a garden room is to choose a garden shed. It’s not going to be as glamorous or fancy as a high-end garden room but you can get some really well-made sheds that will serve your purpose. One thing to note is that the wood of garden sheds is not treated so this is something you would need to do yourself with paint or varnish. If you are planning to use it throughout the year including the winter you would need to insulate the shed yourself using Celotex, mineral wood or styrofoam. Not forgetting an extension lead for electrics and locks for security. It may be basic but you can do the work yourself and personalise it exactly as you want it keeping it within your budget.